In multi-channel digital printing systems, images for a plurality of image channels are printed in alignment onto a receiver medium. In many such digital printing systems, a plurality of printing modules (e.g., inkjet printheads or electrophotographic print engines) are provided, one for each channel, and multi-channel images are printed by moving a receiver medium past each of the printing modules where the channels are printed in sequence. Typically, the different channels (i.e., “image planes”) are used to print different colorants (e.g., cyan, magenta, yellow and black). In some embodiments, a plurality of channels may be used to print a single colorant, or light and dark variations of the same colorant. For example a black colorant can be printed using two different printer channels to increase the density of the printed image. In some embodiments, a first set of channels can be used to print on one side of the receiver medium, and a second set of channels can be used to the print on the opposite side of the receiver medium (using the same or different colorants).
The printed item produced by the digital printing systems need not be restricted to an image printed on the receiver medium for viewing by an observer, but can also include items printed for a functional purpose such as printed circuitry. In this example, the different channels can correspond to different layers in a multi-layer circuit.
In some applications, the receiver medium may undergo changes between the printing of one channel and another. For example, when a multi-color image is printed by depositing ink on a paper-based receiver media, the water in the ink printed for one channel can cause the receiver medium to expand before a subsequent channel is printed. The receiver media could also undergo other processing steps between the printing of the image planes that could change the dimensions of the receiver media. For example, the receiver media could pass through a dryer (in case of printing with liquid inks) or a fusing step (in case of dry powder electrophotography) between the printing of the various channels, which could cause the receiver media to shrink before the printing of a subsequent channel. The desired registration of one channel to another can be adversely affected by the dimensional changes of the receiver media between the printing of the multiple channels. In many cases, the dimensional changes in the receiver may be a function of a variety of factors such as image content of the printed image and environmental conditions.
In another example, a non-conductive layer could be applied over conductive traces for a layer of circuitry printed on a receiver medium before the printing of a subsequent image plane for another layer of circuitry, where the application of the non-conductive layer produces dimensional changes in the receiver media (and the already printed image plane). In such systems, the desired registration of one image plane to another can be adversely affected by the dimensional changes of the receiver media between the printing of the multiple layers.
In some cases, the printing modules used for printing the different channels may have some variation between them, so that there is a dimensional scaling or magnification change between the channels printed by the different printing modules.
In other applications, it may be necessary to adjust the dimensions of a document printed by a digital printing system even if it contains only a single channel. Such an adjustment may be necessary to match the dimensions of the printed document with the dimensions required by a downstream process. For example it may be necessary to adjust the print width of the printed document so that it correlates with the width of a downstream slitting, perforating, or folding operation.
U.S. Pat. No. 4,721,969 to Asano, entitled “Process of correcting for color misregistering in electrostatic color recording apparatus,” teaches a method for correcting color misregistration errors by inserting or deleting an appropriate number of pixels across the width of the image in a uniformly-spaced pattern.
U.S. Pat. No. 5,093,674 to Storlie, entitled “Method and system for compensating for paper shrinkage and misalignment in electrophotographic color printing,” discloses a method for adjusting an image size for a channel of an electrophotographic printer by altering a scanning mirror speed.
U.S. Pat. No. 5,505,129 to Greb et al., entitled “Web width tracking,” discloses a method for tracking the width of a printed medium by detecting the edges of the medium.
U.S. Pat. No. 6,362,847 to Pawley et al., entitled “Electronic control arrangement for a laser printer,” discloses a method for adjusting a length of a printed line by inserting or removing clock timing pulses.
U.S. Pat. No. 6,927,875 to Ueno et al., entitled “Printing system and printing method,” teaches a method for correcting for heat shrinkage by controlling a timing of light emission. The shrinkage is characterized by detecting media edges.
U.S. Patent Application Publication 2007/0172270 to Joergens et al., entitled “Method and device for correcting paper shrinkage during generation of a bitmap,” discloses a method for compensating for paper shrinkage by adding or removing image pixels, preferably in un-inked locations.
U.S. Patent Application Publication 2011/0102851 to Baeumler, entitled “Method, device and computer program to correct a registration error in a printing process that is due to deformation of the recording medium,” discloses a method for deforming an image to correct for registration errors, wherein the pixels to be deformed are selected stochastically.
There remains a need for digital printing systems to correct for any dimensional differences between different image channels that can result from various sources so that the various channels of the printed image can be properly registered.